Method and apparatus for the spray drying of heat sensitive liquiform materials



`lune 22, 1965 P. HussMANN 3,190,343

METHOD AND APPARATUS FOR THE SPRAY DRYING OF HEAT SENSITIVE LIQUIFORMMATERIALS Filed Sept. 28, 1962 2o and 2b Dehumdifying Zones 5 1l .f4 s en o Z su 7 .uw mo 0T S g 1m E u ew v HD 6 .D 3 d n n a 3| United StatesPatent 3,190,343 METHOD AND APPARATUS FOR THE SPRAY DRYENG @Il HEATSENSITVE MQUWGRM MA- TERIALS Peter Hussmann, Florence, Italy, assignorto Birs Beteiliguugsund Verwaltungsgesellschaft AG., Basel, Switzerland,a corporation of Switzerland Filed Sept. 2S, 1962, Ser. No. 227,673Claims priority, application Germany, Get. 5, 1961, is 64,255 30 Claims.(Si. 159--4) The present invention relates to improvements in a methodand apparatus for drying particulate solid materlals, such asdehydrating aqueous dispersione, suspensions or solutions of solids.More particularly, it is concerned with improvements in the dehydratingprocedures disclosed in my copending applications Serial No. 735,299,filed May 14, 1958, for Dehydration Process and Apparatus Therefor, nowabandoned, and Serial No. 87,808, filed February 8, 1961, forDehydration Process, Product Obtained Thereby, and Apparatus forCarrying Out Said Process.

With the procedures disclosed and claimed in these applications, itiirst became possible to obtain instant powders merely by drying. Theinstant powders obtained by these procedures have all the properties ofthe solids contained in the starting material. They are not denaturedand, in the case of food products, they show no change of taste. Theymay be considered as instant products because the powders will instantlydissolve without leaving a residue, even in cold Water, if they arecolloidally soluble or, if they are only dispersible, they will beinstantly dispersed or wetted, while swelling.

In this previously disclosed method, the material to be dried isdispersed in the upper region of a high tower in the form of a denseumbrella of droplets descending in the tower through a counter-currentlyowing stream `of relatively cool or cold drying air, the air beingintroduced in the lower region of the tower in a highly dehumidifedstate, for instance, with a water content of 0.35 g./cu. m., at atemperature not exceeding about 60 C. In view of the low temperature ofthe drying gas, the residence time of the material in the tower must berelatively long and the volume of drying gas must be accordingly large.

It is one of the primary objects of the present invention to increasethe residence time of the materialrto be dried in the countercurrentlyowing drying gas without increasing the height of the drying tower.

It is another object of this invention to operate the drying apparatuswith a minimum of heat requirements.

It is still another object of the invention to handle the dried materialso that it sustains substantially no damage due to denaturing orabrasion until it is packaged in irnpermeable containers.

It is a further object to improve the quality of the dried materialfurther and to control the apparent density of the instant powder.

The above and other objects and advantages may be obtained according tothe present invention due to a singular property of the instant powdersobtained by my cool air drying method. I have found that these instantpowders consist of fully spherical granules with holes reaching deeplyinto the interior of the spheres even when they still contain about 5%to 20% water, depending on the type of material. Once the material hasreached this spherical structure, the powder may be conveyed by airstreams or mechanically without destroying the spherical granules orappreciably abrading the same. Furthermore, an air stream may upwardlypass through a layer of spherical granules of this type and having athickness up to about 20 cm. without causing turbulence in the layer.

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.In one aspect of this invention, special means is prov1ded to preventundue turbulence in such dried goods which should remain as littledisturbed as possible until they are packed and to convey them from thedrying tower to the packing zone in the shortest possible path. If thedried powder is of the type which tolerates stirring, the inventionprovides special means for removing the powder from the drying tower andfor conveying it in the shortest possible path to the packing stationwhich may be, if desired, any conventional vacuum-packing system.

If the starting material is particularly hygroscopic, such as tornatoconcentrate which may still contain tomato peels and which tends tostick to the bottom Vof the drying tower, brushing means must be placedthere to avoid the formation of a solid material layer on the towerbottom. In the various embodiments of this aspect of the invention, thebrushes serve partially not only for conveying the dried material butalso for imparting turbulence thereto.

Different types of apparatus will be used for different materials,depending on their characteristics, the method and apparatus of theinvention being adapted for use with any starting material which may bedried to form a powder. Types of useful apparatus include (l) A dryingtower with a flat or substantially ilat perforated bottom through whichthe drying gas flows upwardly into the tower, the sieve-like bottomtaking a variety of forms.

(2) A drying tower with a conically converging impermeable or perforatedbottom having the material outlet at its apex, with or without brushes.

(3) A drying tower with a hat or substantially flat impermeable bottomwith brushes for imparting turbulence to the powder.

(4) A drying tower with a partitioned jacket to obtain a temperatureequilibrium between the day and night air used as drying gas, on the onehand, and the air dried chemically, for instance, by silica gel ormolecular sieves, so as to store heat and simplify the servicing of thetower.

(5) A drying tower with a special material distributing arrangement forcontrolling the bulk density of the powder.

If desired, the drying tower may be combined with other driers, forinstance, freeze driers for very sensitive materials. Also, aromaticsubstances may be admixed with the drying gas because the porousgranular structure of the dried powder obtained according to my methodmakes it possible for the powder effectively to adsorb aromas.

The above and other objects, advantages and features of the presentinvention will become more apparent in the following detaileddescription of certain preferred embodiments thereof, taken inconjunction with the accompanying schematic drawing wherein In allembodiments of the present invention, use is made of a drying toweraccording to my above mentioned copending applications, wherein thethroughput of the material to be dried, its droplet size anddistribution, the drying gas velocity and temperature are so correlatedthat the material falling through the dehumidied and upwardly streamingdrying gas reaches the tower bottom in the form of porous and fullyspherical granules which remain on the bottom without collapsing oradhering to each other. This novel granular structure of the driedmaterial makes it possible to introduce the gaseous drying agent intothe tower through the bottom, even if it carries a layer of dried powderup to 20 cm. thick and without imparting turbulence to the powder layer.The powder remains lying on the tower bottom until it has reached thedesired degree of dehydration and is then conveyed on the shortestpossible path to the vacuum from the bottom of the tower as soon as itmay be mechanically'or pneumatically conveyed to a` sieve-like supporton which it is additionally dried to the desired degree of dehydrationwithout turbulence.

The drying tower used in the invention may be provided with a falsebottom which is a horizontal porous, sievelike support. l The falsebottom deines a space with'the tower bottom and an inlet pipe supplies agaseous dryingl medium to the-tower. The Vliquid material to Ybe driedin tower. The used drying medium, which carries most or all of the waterfrom the liquid material dried in the tower, may escape into theatmosphere through a ilue or it may be recovered in a pipe leading to agas dehumidifying plant so that the drying medium maybe returned to thedrying tower in a closed cycle after it hasbeen dehuniiditied.Adjustable valves may be mounted in gas outlets to regulate the draft ofthe drying medium according to requirements. Y

The granular powder is collected on the false bottom until it hasreached a desired layer thickness and after it has reached apredetermined degree of dehydration. A scraper, which is rotatable aboutthe axis of the tower, is turned to push the dried material through aradial slo't arranged in the false bottom.Y TheA screw conveyor ismounted ina housing which has an upper opening facing the radial slot sothat the dried material will'fall through the slot Ainto the housing andwill be transported out of the housing and the tower by the screwconveyor directly into a vacuum packing device withoutv contacting theatmosphere. p .v Y

It the scraper has brushes for pushing the dried ymaterial, it may beuseful intermittently to blow a dried gasV through linlet pipe to removematerial adhering Vto the brush bristles,V Adjustable vvalve means aremountedfin inlet pipe to control the gas stream therethrough.

In another embodiment of the invention, the dried ma-` terial is notremoved fromvffalse ybottom by a mechanical scraper but by apneumatic'conveyor. The pneumatic conveyor comprises a radiallyextending pipe arranged for rotation about the axis-of the tower tosweep over fase bottom'with a suction slot facing the falseA bottom'.The radially extending pipe is in communication with an axiallyextending tube whosefoutput'i's connected to the in; terior of acontainer which'is under sub-atmospheric'presdiuxn isintroduced intothe,y tower through inlet opening.V

sure. The tube is rotated through a gear train by a power SOUICe.

If desired, the mechanical andV pneumaticY conveyors Y Y may be socombined that lthe mechanical scraperrpushes the dried material towardsthe circumference of the tower while the pneumatic conveyor has suctionslots only at'the by awplurality of sector-shaped sieve 'boxes'. Theboxes may be turned about their axe-s.r Y Y l The Vbrushes* are alsorotatable about theirax'es by suitable motors and sweepthe undersidesyof thesieve boxes. In this manner, thevdried spherical granules fallfirst on the upper talsebottom, vpassthroiigh its sieve` boxes, areswept oit 'their undersides, fall lon the under-Y lying talse bottoni,pass through the iatters sieve boxes; areswept 'oit their -undersidesand drop to the bottom of Vthe tower wheretliey are swept vby Vawe'ptbyrotating scraper or brush into a central outlet opening through of thetower.

d lwhich the dried powder is'immediately gravity-fed into packingstation without contacting the atmosphere,

Ihedehumidtiedv dryinggas may be introduced into the b-ottom of thedrying Itower a pipe and a baie plate may Ibey mounted over the outletof the pipe to distribute the drying gas over the entire cross sectionof the tower so that it will rise trom the bottom in fa column.

The sieve yboxes may be bounded by two generally horizontallyextendingperforated or sieve-like walls and one of these walls mayhaveplaced tliereover a tine-mesh web. The sieve boxes may be turnedabout their taxes.

The introduction of drying gas is preferably initiated onlyshortly:after the-'sieve boxes have been turned so that the tine-mesh sheet isAfully dried before( another layer of dried'fmaterial is formed thereon.

-In anotherV embodiment of the tower used in the invent-ion,there aremounted two false sieve bottoms in horizontally rectangular crosssection. f

The two talse bottoms are spaced apant and each bottom consists of aplurality of adjacently mounted sieve boxesV bounded by horizonatllyextending sieves covered by tine-mesh sheets. A support extendshorizontally below each talse bottom and carries las brushing device tormovement thereon so as to sweep the underside of each .false bottom.u,Another scraping or brushing device is movably mounted on a supportlabove the-tower bottom for `sweeping the bottom -or door of the towerand con- Vey the ydried materialV to anY outlet opening.

Whiletwo superposed false bottoms may be used, a single false sievebottom will :suffice for materials which may be dried easily. Also, thelower false bottom may cons-ist of a :single sieve instead of theillustrated sieve boxes. In many instances, the scrapingV or brushingde- .|vice maybe omitted at the lower sieve 'bottom because all thedried powder will drop olf the sieve boxes when Ithe same are turned. f

yIn Ianother embodiment of the tower used in the invention, the towermay have Ia false sieve bottom constitiuted Vby a Vperfo-rated conveyorband.4 The bottom of this tower extends laterally to form anafter-drying chamber Whose length is about twice the diameter of thetower. The conveyor is about twice the -diameter of 'the tower. Theconveyor band extendsA into the after-drying chamber and transports thedried material from the tower, through the |after-dryingchamber to thepacking station Without contact Withthe atmosphere. The gaseous dryingmevmove in opposite directions so that the dried material is moved yfromoneV band to the other beforethe last band conveys the material to thepacking station.

`In, another embodiment of ther-tower of the invention, a false sievebottom is rotatably mounted about the axis The drying gas enters thetower through inlet opening in the bottom of the tower and a 4radiallyextending b-aillevplate extends from the center of the .tower to thematerial outlet. vIt desired, ascrew conveyor may be. mounted under,the' bal'lle plate to transport the dried material ,out of the tower tothe packing station. Alternatively, adried gas may be used toblowthedried v"material into material outlet, to which suction may be appliedi-f needed or desired. A scraping or brushing device is larrangedabovethe bafile plate to sweep dried material olf the plate'and onto,the sie-ve bottom. In anotherdry- AingY tower in accordance'with theinvention a false sieve bottom maybe yieldingly mounted on the towerwall by means of spring supports. The spring supports may be excitedbyan oscillating device so that the spring-suspended bottom isoscillfate-dl andk dried material is passed therethrough to theY bottomof the tower. I The underside of the Asieve bottom is swept by ascrapingor brushing device rotatable about the axis of thetower. Asimilarrotating `scraping or brushing'device sweeps'over'the botythere into thetower itself. ymaterial losses are encountered because any nes are tomor floor of the tower `and conveys the dried material to outlet opening.

Another embodiment of the tower of the invention may have a conicalbottom converging towards a central outlet opening leading to anafter-drying chamber wherein three perforated conveyor bands are mountedone above the other.

In another embodiment, the tower may be itted with an imperforate andalmost fiat false bottom. This ernbodiment is particularly useful forvery hygroscopic materials or materials of different components, such astomato concentrate containing tomato peels. The peels dry more slowlythan the remainder of the concentrate and tend to clog any sieve afterextended operation. Therefore, it is necessary to dry such a material toa large extent on an imperforate support before drying is completed on asieve.

An imperforate false bottom may have the configuration of an annulartrough with only slightly inclined side walls.

In accordance with the invention, the dehumidified drying gas is blowninto the tower through an inlet pipe whose vertically extending portionsurrounds a drive shaft and leads to a baie plate which deilects thedrying gas in the direction of the rotating brushes. A brushing deviceis mounted above the baflie plate to sweep the same and remove therefromany dried material and drop it to a false bottom.

A stirrer may be rotated by a motor to move the dried material through asieve in the after-drying chamber. If the dried material is not toosensitive to abrasion, the sieve in the after-drying chamber may beoscillated for more rapid removal of dried material therefrom. Usefuloperating conditions of this embodiment of thc invention include a speedof two to ten rotations per minute of the brush elements about the toweraxis while they are rotated about their own axes at Such high speeds as300 to 1400 r.p.m. In this manner, the brushes serve not only for themechanical conveyance of the dried material from the false bottom of thetower but also keeps the material from adhering to the false bottom andmaintains it in a turbulent state. In view of the high rotational speedof the brushes, it is preferred to use nylon bristles or strips. Likesynthetic resins may be used instead of nylon.

In another embodiment the drying tower may have a slightly conicalimperforate false bottom converging toward central material outletopening. The false bottom is swept by a brushing device which conveysthe dried material into an opening. An inlet pipe for a dry gascontaining a suitable aromatic substance leads to a radially extendingpipe adjacent each brush and having a downwardly open outlet slot. Inthis manner, the porous spherical granules on the bottom will beimpinged directly by the aroma-containing gas and will adsorb the aroma.The drying gas proper may be introduced into the tower by an inletconduit whose vertical portion coaxially surrounds a pipe and leads to abattle plate which detlects the drying gas and distributes it uniformlyabout the false bottom of the tower. A funnel is rotatably `mounted onthe pipe inlet and leads from the outlet opening to a selected one ofthree after-drying charnbers. The three after-drying chambers may be sooperated that one is continuously filled with dried material coming fromthe tower while the second one is used for after-drying and the thirdone is emptied. In such an operation, the drying gas is first introducedinto the chamber used for after-drying whence it is led into thecharnber filled with the material from the tower and from ln such anoperation, no

returned to the tower by the drying gas and are removed from theupwardly streaming drying gas by the liquid material sprayed into thetop of the tower.

Enrichment of the dried material with an aromatic CIK substance has beenparticularly useful in the produc'- tion of instant coffee. With the useof different aromatic substances, coffee of different quality may beobtained in this manner. For instance, enrichment of the coffee aromamay be obtained with the gas obtained from the end phase of the coffeeroasting just at the beginning of point at which the coffee beans turnbrown. Alternatively, the gases following the cooling of the coffeebeans after roasting may be used for this purpose or a gas Conductedthrough the coffee while it is ground. Finally, use may be made of thegases evolving during any stage of extraction. In each instance, theinstant coffee will have a different aroma.

Since the drying method of the invention with relatively cool air of theinvention requires drying towers of large volume and correspondinglylarge volumes of drying gas, the present invention also provides foroperation of the drying tower with as little heat requirement aspossible and with a maximum recovery of the required heat. For thispurpose, the drying tower is connected with a special air conditioningplant.

Drying towers useful for the practice of this invention must have aheight in the range of 50 m. to 200 m., tower heights of about 70 m.having been found most useful for most purposes.

In accordance with the present invention, a minimum of structural unitsare used for the heat recovery and the dehumidification of the air andthese units for conditioning the drying air are part and parcel of thedrying tower. By placing the dehumidification housing into the dryingair and heating cycle, the invention maintains a constant temperature ofthe drying air entering into the tower over each cycle ofdehumidification and regeneration of the drying medium, with no, or aminimum of heating or cooling of the drying air. This temperatureequilibrium is obtained inside the drying housing in multiple steps. Ifdesired, the drying air may be additionally heated in the drying towerfrom the walls of the tower so that the air keeps its entry temperaturewhile streaming upwardly in the tower and thus maintains its dryingpower almost until it reaches the region where the liquid material isreleased.

The dehumidification of the drying air is effected with the aid ofadsorption media, such as silica gel or molecular sieves and experiencehas shown that a packing of such adsorption media may remove water fromthe air passed therethrough for about 5 to 6 hours. This must then befollowed by a regeneration cycle of about equal duration during whichhot air of constant temperature is passed through the adsorption mediumpacking, the temperature of the regenerating air ranging between aboutC. and 300 C., depending on the adsorption medium.

The cold or cool air drying of the present invention requires a constantsupply of dehumidified air of a temperaturc of about 25 C. to 30 C., forinstance, for introduction into the drying tower and of about C. forregenerating the dehumidifying packing if silica gel is used for thispurpose, for instance. Thus, the invention provides improved means forconditioning the air in this manner with a minimum of servicing.

This is accomplished in accordance with the invention by passing thefresh air through a chamber defined by the outer wall of the dryingtower and a heat-storing intermediate wall between the outer and innerwalls of the tower, while the dehumidified fresh air is passed throughthe chamber defined by the inner tower wall and the intermediate wall,which has an inwardly facing heatinsulating lining, after it has beenconducted through the dehumidifying adsorption medium and before it isintroduced into the drying tower. If additional cooling is required,fresh air for regenerating the adsorption medium may also be passedthrough the first-named chamber on its way to the adsorption mediumand/or through a heatstorage device in the regenerating cycle.

perature difference of the atmospheric air'between night" and day time,on the one hand, and the rise of the temperature of the adsorptionmedium during the to 6 hours dehumidifying cycle from about to 20 C. toabout' 40 to 50 C. When therfresh Vair is passed through the outerchamber 'of the double wall or jacket ofthe drying Y tower during vthenight, the heat-storing intermediate wall in the jacket will be cooled,together with the heat-` storing outer wall of tower, to such a degreethat the fresh air conducted to the adsorption medium will Anever exceeda maximum temperature of about C. during the entire day. The lower thetemperature of the air passing through the adsorption medium packing,the higher the dehumidication eciency of the packing. In the initialtime period of dehumidification, the dried air leaves the packing withan average temperature of about C. In the preceding cycle correspondingto the final time period of dehumidication, the dried air left theadsorption medium at a temperature of about 50 C. and caused the innerwall of the drying tower jacket to be heated to an average temperatureof 45 C., as the said air is passed through the inner jacket chamberbefore introduc-A tion into the drying tower. Heatexchange accordinglycauses the initially dehumidied air to be heated to C. during itspassage through the inner jacket chamber on its way to the drying tower,atewhich temperature it then enters the interior of lthe tower. In thesecond period ofdehurnidiiication, the dried air leaves the packing withVan averagetemperature of about 35 C. and this rises to C. or so in thetinaldehumidication period. The

warmer air raises the temperature of the previously cooled Y tower wallso thatheat exchange continues to keep the temperature of thefdehumidified air entering the drying tower at about 35 CQ This cycle isrepeated asvadditional fresh air is passed through a secondY adsorptionYmedium packing which has, meanwhile,- been regenerated.

Temperature control may be obtained by imparting a zigzag configurationto thel intermediate wall inthe drying towerY jacket, which `increasesits Vheat-storage capacity andY also sub-divides the space between theVouter and inner tower walls into individual cells. The heat exchangemay then be controlled byrsuitable control of the passage and amount ofair streams into the individual cells. Thus, a variety Vof drying airtemperaturesmay be obtained independently of the temperature of the"atmospheric air, in addition to the temperature of 35 C; givenhereinaboveby way of example. In case of rextreme climatic conditions orspeciallyrlowdrying 'air'ternperatures required for certain materials,for instance, 18V C.

for drying butter, specialV heatexchangers for cooling or lheating thefresh air may be positioned inthe fresh vair conduits ahead of theadsorption medium or the yinlet into the drying tower. In rn'ostVinstances, however, it is posv Y `aros. 1 amamantar@ a sV f Y structuralembodiment of .the plant for conditioning the drying air -in theindicated man- Y ner, fFIG. 1 being a vertical section of the tower andFIG.

2 show-ing a horizontal section thereof.' To facilitate a schematicshowing of the conduits, theV elements 2a, 2b, and 3a, 3b are .shown inFIG. 1 one ybehind theV other, while they are illustrated adjacent oneanother in FIG. 2. In practicethey are-arranged only at one ofthe twolocations. Elernents '6 land A7 also are positioned atthe samelocation.l

The drying 'air enters the drying tower at inlet opening 9 and passesupwardly to outlet opening 11;" The liquid material distributing means'1(n`ot' shown)'is mounted in the 'tower below the drying gas outletopening. The tower,

VVwhich may have circulary or rectangular walls, is surrounded by ajacket formed by 'an inner and` outer' tower wall defining a spacetherebetween. This space` is divided into outer .cells 4 and inner cells5 by an intermediate zig- Y the adsorption mediumis passedv into theplant fromthe atmosphere through theA outer cells 4 ofthe tower jacket.

YIf desired, the vair may be led upr and rdown the cells several times`lbefore lleaving the jackets. For instance, it may be blown into thecells 4 of one side wall of the tower by eight fans mounted in frontfofsuitable Vinlet openings in the wall.rv It may then be directed, forinstance from the lower -ce-lls of the one wall into the outer cells 4ofthe two adjacent walls, passing .upwardly through these cells andbeing conducted by suitable conduits from the upper cells into; theouter'cclls of the adjacent wall opposite to the `first-named wall,whence the air-is led through openings v in the lower pontion of thelatter wall into la header.

sible to dispense with such special heat exchangers or to operate thesame only'for short periods of time at certain Y pointsjof the cycle.

.Since `the temperature of the dehumidicati'on packing rises at 4theendv of the dehumidication and regeneration cycles, suitablethermocouples may `be used for automati-` cally switching the'packingsfrom-the dehumidication to the regeneration cycle,and vice versa. VIfdesired-,- the entry temperature ofthe drying gas into the' drying towermay lbe used to initiate this switching a little sooner, particu--Vlar-ly if the 'latter temperature V'is also used to direct the dryingai-r suitably through the individual jacket cells and `to control itsthroughput .to obtain the desired drying gas temperature. If athermostat indicates this desired tern` peratur-e to be exceeded, it maylthen automatically switch the gas `stream through 'a freshlyregeneratedads'orption medium while causing regeneration of theexhausted rnc-V dium. The entire vair, circulation -throughthedryingplant Vmay be eifected by fans mountedin the fresh air inlet open- Yings of the outer wall of the drying tower.

As'shown in iEIG.V 1, the header is sub-divided into two branchconduits. In oneV of the operating cycles,'the fresh air, which hasVbeen pre-heated by passage through jacket cells 4,1 is fled through.the upper branch conduit into a lregenerated :adsorption medium packing2a consisting of two vertical packingsarranged in parallel. The otherpor- 'tgion of thev pre-heated `fresh airis conducted into a heatstorage device y3a. `The lentryvvalveY of the lower branch .conduit thenremains closed. In the followingoperating cycle, Va portion .of thefresh air coming from cells 4 is conducted by the lower branch conduitto adsorption medium packing 2b which 'is similar to .structure topacking 2a; The other portion of the fresh air `is then led from valveof the 'upper Ibranch circuit VDepending on the'cycle, the dehumidiedair coming from packing 2a orb is conducted to the inner cells 5 of thetower jacketand is led up and down thesejcells Vin a manner Vsimilar tothat described in connection with the passage ofthe fresh lair throughcellsY 4. Finally, the de- 'humiditied and conditioned drying airis'introducedv into the tower through opening 9. f

The fresh vair' further: heated in heat storage devices 3a or. Sbis ledthrough the flue of the heating element of another heat exchanger 6,Iwhere it is additionally heated, Iand isy finally directed into 1 aheat exchanger -7 where it is Abrought to the required: regenerationtemperature. This hot air is ythen directed to the adsorption mediumpacking requiring'regeneration, whence ilt is passed into one of heatstorage ydevicesrz -or 3b to heattheysarne. The 4regenerating airthen-passes from the heat storage :devices into the atmosphere throughiiue 10. The inlet openings for cells 4, which admitithe fresh :airthereto, and for cells 5,

provided with :suitable filters.

If the temperature of the adsorption packing used for dehumidifying theair exceeds a predetermined value, for instance 50 C., a thermosensitiverelay may automatically operate suitable valves in the air conduits todirect the fresh air to a previously regenerated adsorption packing andto direct the regenerating air to the exhausted packing. The control -ofthe next cycle may then be taken over by a thermostat in the-regenerated packing. However, the cycle control Kmay also be effectedin an obvious manner by a thermostat in the conduit directing thecondtioned air to cells 5.

By -using a .tower wall jacket as heat storage and balancing device, itis possible -to equalize the air temperatures so as to obtain a dryingair of nearly constant temperature throughout the operation of thetower. Since the atmospheric air is first led through outer jacket cells4, its temperature may `be raised sufliciently on its way to theadsorption packing so that the conditioned and dehumidified air willnever drop below a given temperature, for instance 2-5 t-o 30 C., at itsentrance into the tower. Temperature control may be achieved by creatingzones 4of different temperatures in the tower jacket. -For instance,

the fresh air supply fans, which may be eight or nine in number, forinstance, may each supply a separate air conduit leading to the headerso that, depending on the fan speed and the corresponding air throughputof each fan, the air entering the adsorption packing from each conduithas a different temperature. In a similar manner, the drying air may `be`prevented from reaching a temperature higher than desired by conductingthe air on 4its way to the tower through more or fewer cooled cells 5.Excess air temperature toward the end of the dehumidifying cycle mayalso be avoided by switching prematurely to the subsequent cycle as soonas the temperature reaches a certain point if it would `be impossible tochange the air path through the jacket cells. This switch may beautomatically etiected by a thermostat at inlet opening 9.

Since the time period during which drying air is introduced into thetower at a temperature substantially exceeding the desired temperaturelevel is relatively short in each cycle, the inner wall of the towerjacket will be maintained `at substantially constant temperature. Thismay be accomplished by suitable dimensioning the wall thicknesses.Furthermore, additional control may be obtained, if desired, by liningthe outwardly facing surface of the intermediate jacket wall with aninsulating layer in `some of the `jacket cells. Heat storage is effectedprimarily in the zigzag intermediate wall in the tower jacket. It isalso possible to make the Volume of cells 4 and 5 different, forinstance to make the cells 5 larger than cells 4. The amount `of airpassing through cells 4 may be one and a half or two times that passingthrough cells 5, depending on the ratio of drying ai-r to regeneratingair, so that the cool night air may be used to an increased extent forcooling the heat exchangers.

As indicated hereinabove, the adsorption medium regenerating air is notled through cells 5 but is passed into the atmosphere at it). The risingdrying air tends to decrease in Itemperature from the bottom to the topof the tower and if desired, the jacket `space may be subdivided byhorizontal partition walls so that the inner wall of the tower jacketwill be kept at different temperatures at different levels.

Some water will be removed from the fresh .air by condensation as itpasses through outer jacket `cells and a suitable wa-ter outlet isprovided at the bottom of cel-1s 4 in the jacket.

While the invention has been described in connection with certain nowpreferred embodiments, it will `be clearly understood that manyvariations and modifications may occur to be `skilled in the art,particularly after beneiitt-ing from `the present teaching, withoutdeparting from the spirit and scope of the present invention as definedin the appended claims.

I claim:

1. The method of supplying air into a drying tower having a jacketformed by an inner and an outer tower wall defining a spacetherebetween, said space being divided into a first and a second regionby an intermediate wall and the walls of the iirst region having beenpreconditioned by heat-exchange with atmospheric air at one temperature,which comprises subsequently passing atmospheric air having anothertemperature through the rst region to exchange heat with the walls ofthe first region; then dehumidifying the air after it has left the firstregion by feeding it through a dehumidifying zone; then passing thedehumidiiied air through the second region to exchange heat with thewalls of the second region; and then feeding the dehumidiiied air havinga temperature not exceeding about 60 C. into the drawing towercontinuously for at least one diurnal period.

2. The process of claim 1 in which the temperature of the dehumidifiedair which is fed into the tower does not exceed 30 C.

3. The process of claim 1 in which the dehumidifying zone has atemperature in the range of 15 to 50 C.

d. The process of claim 1 in which the dehumidilied zone has atemperature in the range of 20 to 40 C.

5. The method of supplying air into a drying tower having a jacketformed by an inner and an outer tower wall defining a spacetherebetween, said space being divided into a first and a second regiondivided by an intermediate wall and the walls of the first region havingbeen preconditioned by heat-exchange with atmospheric air at onetemperature, which comprises cooling atmospheric air having a highertemperature in the first region by releasing heat to the walls of thefirst region; then dehumidifying the cooled air by feeding it through adehumidifying zone; then passing the dehumidifying air through thesecond region to exchange heat with the walls thereof; and

feeding the dehumidified air having a temperature not exceeding 60 C.into the drying tower.

6. The method of supplying air into a drying tower having a jacketformed by an inner and an outer tower wall defining a spacetherebetween, said space being divided into a first and a second regionby an intermediate wall and the walls of the first region beingpreconditioned by heat-exchange with atmospheric air at one temperature,which comprises heating atmospheric air having a lower temperature inthe first region by absorbing heat from the walls of the first region;then dehumidifying the heated air by conducting it through adehumidifying zone; then passing the dehumidified air through the secondregion to exchange heat with the walls thereof; and feeding thedehumidied air having a temperature not exceeding 60 C. into the dryingtower.

7. The method of supplying air into a drying tower having a jacketformed by an inner and an outer tower wall defining a spacetherebetween, said space being divided into a first and a second regionby an intermediate wall and the walls of the first region having beenprecondtioned by heat-exchange with atmospheric air at one temperature,which comprises heating atmospheric air having a lower temperature inthe first region by absorbing heat from the walls of the first region;then dehumidifying the heated air by conducting it through adehumidifying zone;

heating the dehumidified air by absorbing heat from the walls of thesecond region;

feeding the heated dehumidied air having a temperature not exceedingabout 60 C. into the drying 'i l l tower, then cooling the dehumiditiedair by .releasing its heat to the walls of the second region and feedingthe dehumidiied air having a temperature not exceeding about 60 C. intothe drying tower. 3. The method of supplying air into a drying towerhavalemania ing a jacket formed by an inner and an outer tower .wall

defining a space therebetweenfsaid space being divided into a rst and asecond region by an intermediate wallV and the walls of the first regionhaving been preconditioned by heat-exchange with atmospheric air at onetemperature, which comprises .f f cooling atmospheric air havinga highertemperaturein the first region by releasing heat to the walls of thefirst region; then Y dehumidifying the-.cooled air by feeding it througha dehumidifying Zone; then Y v cooling the dehumiditied air by releasingheat to the walls of thesecond region; feeding the cooled dehumidied airhaving a temperature not exceeding 60C. into the drying tower, thenheating the dehumidied air by absorbing heat from the walls of thesecond region and feeding the dehumidi-V tied air having atemperaturernot exceeding about 60 v C. into the drying tower. 9. Themethod of supplying air continuously ceeding about 60 C. into a dryingtower having a jacket formed by an'inner and; an outer tower walldeiining a` space therebetween, said space being divided into a rst Y isecond region by an' intermediate wall and the Walls of*l the firstregion being preconditioned by heat-exchange with atmospheric air at onetemperature, which comprises Y cooling atmospheric day air-havingahigher. temperature in theV rst region by releasing its heat totherwalls Y of the lirst region; then dehumidifying the cooled air byconducting it through a dehumidifying zone; then passing the dehumidiiedair through the second` region to exchange heatwith the walls of theVsecond region;` then feeding the dehumiditied air having a temperaturenot exceeding about 60 fC. into the drying tower;

heating atmospheric nocturnal air in the tiret' region by absorbing heatfrom the walls of the'rst region; then l dehuinidifying the warmed airby conducting it through ing a jacket formed by an inner and anouter'towervwall defining aV space therebetween, said space beingdividedY into a first and a second region by an intermediate wall andthe walls of the irst region being preconditioned by for atleast l onediurnal period at a temperature in the range notex- :iaY

defining a space therebetween, said Vspace being divided into a iirstyand asecond region by an intermediate wall and the walls of the rstregion being ,preconditioned by heat-exchange rwith atmospheric air atyone temperature,

which comprises (a) during the day y. cooling atmospheric air having ahigher temperature in the fiirstV region by releasing its heat to thewalls of the tirst region; then dehumidifying the air by feeding itthrough a dehumidifying Zone; then Y n passing the dehumidiiied airthrough the second region to exchangeV heat with the walls thereof,andthen feeding the dehumidiied air having a temperature not exceedingabout 60 C. into the drying tower continuously, and

(b) duringthe night Y heating'atmospheric airrhaving a lower temperaturcinthe first region by absorbing heat from the wallsfof ythe firstregion; then f dehumidifyingthe air by feeding itthrough a de- YVhurnidifying zone; then Vpassing the dehurnidiie'dl air throughthesecond region to exchange heat with the walls thereof;

and Y feeding the dehumiditied air having a temperature not exceedingabout 60 C. into the drying tower r v continuously. i2'. The method ofsupplying-air into a drying tower having a jacket formed by an'innerfandan outer tower wall defining a space therebetween, saidvspace beingdivided into a-first and a second region by an intermediate wall and thewalls of the iirst region being preconditioned by heat-exchange withatmospheric'air at one temperature, which comprises y passingatmospheric air having another temperature throughthe. first region toexchange heat with the Vwalls of the lirstregion; f y dehumidifying theairafter it has left the yiirst region y f by feeding itV through-adehumidifying zone; then subjecting the dehumidied air to an alternatingheating and cooling cycle during which' heating cycle, the dehumiditiedair is heated Vby absorbing heat from the walls of the second region andduring .which cooling cycle, `the dehumidiiied air is cooled byreleasing heat to the walls'of the` second region; and then feedingdehumidilied airhaving'a temperature not ex- Y eeeding about V60 C. intothe drying tower.

i3. The processof claim y12 -in which the air which is fed into thedrying Ytower is essentiallyrkept ata constant f temperatureV notexceeding about 60 C.

1d. The method of supplying airv continuously for at least one diurnalperiod at a temperature not exceeding about 60 C.' into a drying, towerin which material to be dried is descending from the upper region of'vthe tower pas a dense umbrella of dropletsy and is accumulating as aheat-exchange with atmospheric air at one temperature,

which comprises Y y i passing atmospheric air havingV anotherVtemperature through the iirst region to exchange .walls of the firstregion; then dehumidifyingthe air by feeding itlthrough a dehumidi-Vhreat with the tying zone which has a dehuniidifying and a regenjeration cycle;

y passing 'the'dehumidiied airiithrough the second regionV to'exchangeheat with the wallsthereof; andi. l feeding thedehumidiiied airatatemperature substantially constant and not exceeding about 60 C;during Y each cycle of the dehumidifying apparatus `continuously intothe drying tower for at leastl one diurnal layerrof dried material onthe bottom of the tower, said drying tower having a jacket formed by aninnerv and an outerV tower Vwall deiining'a space therebetween; saidspace being divided into a first andra second region by an intermediate,wall and the walls of the ir'st regionbeing preconditioned byheat-exchange with atmospheric air at one temperature, which comprises Yv v v passing atmosphericair havinganother temperature vthrough theiirst region to yexchange heat with the walls ofthe first region; Y Y Adehumidifying'the air after it has lett the iirst region byfeeding itthrough. a dehumidifying zone; then vpassing the dehumidiiied airthrough `thersecond region to exchange heat with-the wallsyof the secondregion; Vandthen i Y feeding thedehumidiied air'having a temperature notexceedingabout 601`C. into the drying tower; and

feeding the air upwardly within the drying tower for at least onediurnal period. 15. The process of claim 14 which comprises feeding thedehumidiiied air into the bottom of the tower and then upwardly throughthe layer of pre-dried material accumulated at the bottom of the tower.

16. The process of claim 14 which comprises feeding the dehumidiiied airupwardly through a layer of pre-dried material accumulated at the bottomof the drying tower, then upwardly as an airstrearn conntercurrently andthrough the umbrella of droplets of material to be dried descending inthe tower.

17. The tower of claim 15 which has air-conduct means interconnecting aplurality of the cells.

18. The method of supplying air into a drying tower having a jacketformed by an inner and an outer tower wall defining a spacetherebetween, said space being divided into a first and a second regionby an intermediate wall which comprises preconditioning by heat-exchangethe wails of the first region with atmospheric air at one temperature,

passing atmospheric air having another temperature through the firstregion to exchange heat with the walls of the first region;dehumidifying the air after it has left the first region by feeding itthrough a dehumidifying zone; then passing the dehumidified air throughthe second region to exchange heat with the walls of the second region;and then feeding the dehnmidied air having a temperature not exceedingabout 60 C. into the drying tower continuously for at least one diurnalperiod.

19. A drying tower using dehnmidified atmospheric air as a dryingmedium, said tower comprising a heat-exchange jacket surrounding thetower, said jacket being deiined by an outer and an inner Wall ofheat-exchanging material, an intermediate wall of heat-exchangingmaterial in said jacket, said intermediate wall dividing the jacket intotwo chambers adjacent the outer and inner wall, respectively.

20. The drying tower of claim 19 in which the thickness of theintermediate wall exceeds that et the inner and of the outer towerwalls, respectively.

21. The tower of claim 19 which has a heat-insulating layer on theintermediate wall on the side facing the outer tower wall.

22. The tower of claim 19 which has a heat-insulating layer on theintermediate wail on the side facing the inner tower wall.

23. The tower of claim 19 in which the intermediate wall divides eachchamber into a plurality of cells extending along said walls.

24. The tower of claim 23 in which the intermediate wall has a zigzagconiguration.

25. The tower of claim 19 in which the two chambers have a dierentvolume.

26. The tower of claim 25 in which the volume of the inner chamberexceeds that of the outer chamber.

27. A drying tower using dehumidified atmospheric air as a dryingmedium, said tower comprising a heat-exchange jacket surrounding thetower, said jacket being defined by an outer and an inner wall ofheat-exchanging material, an intermediate heat-insulating wall ofheat-exchanging material in said jacket, said intermediate wall dividingthe jacket into two chambers adjacent the outer and inner wall,respectively.

28. A system which comprises a drying tower using dehumidied atmosphericair as a drying medium, said tower comprising a heat-exchanging jacketsurrounding the tower, said jacket being defined by an outer and aninner wall of heat-exchanging material, an intermediate wall ofheat-exchanging material in said jacket, said intermediate wall dividingthe jacket into a first chamber adjacent the outer wall and a secondchamber adjacent the inner wall and dehumidifying means, the twochambers, the dehumidifying means and the tower being so operativelyconnected that during operatic-n of the tower atmospheric air passesthrough the first chamber, then through the dehumidifying means, thenthe second chamber and then into the bottom part of the tower at atemperature not substantially exceeding 60 C.

29. The system of claim 28 in which the dehumidifying means comprise twozones, one for dehumidifying the air as it passes from the first to thesecond chamber, and a second zone that undergoes regeneration as the rstzone dehumidies the air.

30. A system which comprises a drying tower using dehumidifiedatmospheric air as a drying medium, said tower comprising aheat-exchanging jacket surrounding the tower, said jacket being definedby an outer and an inner wall of heat-exchanging material, anintermediate heat-insulating wall of heat-exchanging material in saidjacket, said intermediate wall dividing the jacket into a first chamberadjacent the outer wall and a second charnber adjacent the inner walland dehumidifying means, the two chambers, the dehumidifying means andthe tower being so operatively connected that during operation of thetower atmospheric air passes through the first chamber, then through thedehumidifying means, then the second chamber and then into the bottompart of the tower at a temperature not substantially exceeding 60 C.

References Cited bythe Examiner UNITED STATES PATENTS 1,289,779 12/ 18Howard.

1,730,048 10/29 Zizinia 159-4 1,870,423 8/32 Richards.

1,946,566 2/ 34 Bowen.

1,959,301 5/ 34 Northcutt.

2,253,319 8/ 41 Batterman.

2,494,644 1/50 Clement 34-80 X 2,584,573 2/52 Gay 165-18 X 2,589,2623/52 Keith.

2,621,966 12/52 Jehlicka.

2,635,684 4/53 Joscelyne.

2,776,562 1/57 Davie et al 16S-4 X 2,842,193 7/ 58 Ballestra.

2,851,097 9/5 8 Ledgett.

2,990,011 6/ 61 Stratford.

3,007,254 11/61 Schuster 34-35 X NORMAN YUDKOFF, Primary Examiner.

11. THE METHOD OF SUPPLYING, FOR AT LEAST ONE DIURNAL PERIOD, ACONTINUOUS SUPPLY OF AIR INTO A DRYING TOWER HAVING A JACKET FORMED BYAN INNER AND AN OUTER TOWER WALL DEFINING A SPACE THEREBETWEEN, SAIDSPACE BEING DIVIDED INT A FIRST AND A SECOND REGION BY AN INTERMEDIATEWALL AND THE WALLS OF THE FIRST REGION BEING PRECONDITIONED BYHEAT-EXCHANGE WITH ATMOSPHERIC AIR AT ONE TEMPERATURE WHICH COMPRISES(A) DURING THE DAY COOLING ATMOSPHERIC AIR HAVING A HIGHER TEMPERATUREIN THE FIRST REGION BY RELEASING ITS HEAT TO THE WALLS OF THE FIRSTREGION; THEN DEHUMIDIFYING THE AIR BY FEEDING IT THROUGH A DEHUMIDIFYINGZONE; THEN PASSING THE DEHUMIDIFIED AIR THROUGH THE SECOND REGION TOEXCHANGE HEAT WITH THE WALLS THEREOF, AND THEN FEEDING THE DEHUMIDIFIEDAIR HAVING A TEMPERATURE NOT EXCEEDING ABOUT 60* C. INTO THE DRYINGTOWER CONTINUOUSLY, AND (B) DURING THE NIGHT HEATING ATMOSPHERIC AIRHAVING A LOWER TEMPERATURE IN THE FIRST REGION BY ABSORBING HEAT FROMTHE WALLS OF THE FIRST REGION; THEN DEHUMIDIFING THE AIR BY FEEDING ITTHROUGH A DEHUMIDIFYING ZONE; THEN PASSING THE DEHUMIDIFIED AIR THROUGHTHE SECOND REGION TO EXCHANGE HEAT WITH THE WALLS THEREOF; AND FEEDINGTHE DEHUMIDIFIED AIR HAVING A TEMPERATURE NOT EXCEEDING ABOUT 60* C.INTO THE DRYING TOWER CONTINUOUSLY.